1. Field of the Invention
This invention relates generally to the field of telescopes, and more particularly to systems and methods for telescope guiding.
2. Description of the Related Art
Images of celestial objects are typically captured using a telescope, to which an imaging device such as the charge-coupled device (CCD) array of a camera has been coupled. In some cases, particularly when the object is dim, a long exposure time is required to gather enough light to form a useful image. When this is the case, it is necessary to adjust or “guide” the position of the telescope during the exposure, to ensure that the object remains accurately positioned on the imaging device. “Clock drives” which compensate for the earth's rotation are commonly employed; however, occasional correction is typically required to maintain a desired accuracy.
Many techniques have been employed to provide the necessary guiding. One common method involves the use of a “guide” scope which is attached to the main telescope. The guide scope is used to find a bright “guide” star, and a means is provided for repositioning the telescope as needed to keep the guide scope locked onto on the selected guide star, thereby keeping the main scope aligned on the object of interest.
One possible guiding technique is described in U.S. Pat. No. 5,525,793 to Holmes et al. Here, an “imaging CCD” and a “guide CCD” are positioned on a common focal plane. The guide CCD is used to lock the telescope onto an off-axis guide star, while the imaging CCD accumulates a long exposure image, which can take place over a period of minutes or hours.
This technique generally works well, but suffers from several problems. For example, in some cases, a filter is employed in front of the imaging CCD, in order to capture one component of a color image. However, with both CCDs on a common focal plane, the filter will also be in front of the guide CCD, thereby reducing the spectral passband of the light reaching the guide CCD. This requires the selected guide star to be bright enough to get through the filter, thereby reducing the pool of useable guide stars. The same is true when using a passband filter for photometry purposes.
Another problem is that, at longer focal lengths and larger F/numbers, good guide stars are relatively uncommon, forcing a user to either offset his imaging CCD from the object being imaged to find a guide star, or to rotate the whole assembly to find a guide star. Both are detrimental to astrophotography, since the imaging CCD is expensive in large sizes, and it is frustrating to have to lose the proper framing of a scene to find a guide star, effectively wasting a portion of the CCD area.
One approach to overcoming the problems described above is to locate the guide CCD in a separate housing in front of the filter, and use a beamsplitter or pickoff mirror to direct light from a guide star received via the main scope's aperture to the guide CCD. However, with this arrangement, the guide CCD still only views a small area of sky, and requires offsetting or rotating the main CCD to find a guide star. Another problem in practice is that, if filters are used with different thicknesses, the user must refocus the guide CCD when changing filters.
Another solution is to locate the guide CCD in a separate housing on a guide telescope parallel to but offset from the main telescope tube. This allows the guide scope to receive light without any intervening filters, and to have a short focal length and fast F/ratio, making it much easier to find a guide star. However, the problem with this approach is that any mechanical deflection of the guide scope relative to the main scope will cause guiding errors, resulting in elongated stars and ruining the image. A guiding error of as little as one arc-second—which is a mechanical displacement of only 60 micro-inches across a 12 inch baseline, typical of guide scope support clamp spacing—can be deleterious. As the main telescope is rotated to follow a star, the gravity vector shifts and the support structure can deflect—and even the main telescope tube can bend slightly—shifting the image by an amount much greater than one arc-second.
One other problem should be noted. Most telescope mounts are massive, supporting hundreds of pounds of telescopes and counterweights, and have difficulty making fast, accurate correction of the star image position on the imaging CCD. To avoid having to move the entire telescope when just a fine adjustment is needed, users can employ an “active optical” (AO) device. Such devices, such as a tip-tilt mirror or a tilting thick glass plate, are positioned between the imaging CCD and the main telescope's aperture, and are moved as needed to achieve responsive guiding. However, this arrangement is not possible when a separate guide scope is used, since the guide CCD does not view the sky through the AO device. Thus, the solution that makes finding guide stars easiest cannot be used to control an AO device in the closed loop manner that is required for accurate guiding.